Dry Eye Disease (DED) has been a serious public health issue, with symptoms including discomfort, visual disturbance, and irritation that may cause damage to the ocular surface. However, the understanding of the mechanisms underlying DED is still at an early stage. According to some, it is a prerequisite that the normal tear film be better understood if we are to advance our ability to effectively manage DED. The tear film is the ocular surface fluid that contributes to keep the cornea healthy and functional, and as such it plays a critically important role in keeping normal visual function for the ocular optical system. The normal tear film consists of three layers: the lipid layer, the aqueous layer, and the mucin layer. The lipid layer is secreted by the meibomian gland and is about 20˜150 nm thick; underneath the lipid lies the aqueous layer, which contributes the largest volume to the tear and is about 3˜7 microns thick; the mucin layer is the interface between the aqueous layer and the cornea, which creates a rough interface between the cornea and the aqueous layer. The rough interface serves to attach tears to the corneal surface.
Tear film instability, which is quantified as the temporal thinning of the tear film thickness leading to tear film breakup, has been established as a core mechanism of DED by some. Tear film thickness can be measured using both invasive and non-invasive methods. Since the invasive methods disturb the tear film in the measurement procedure, non-invasive methods have been sought. In 1989, Doane pioneered the development of a non-invasive interferometric method to measure the tear film (see M. G. Doane, “An instrument for in vivo tear film interferometry,” Optometry Vision Sci. 66 (6), 383-388 (1989)). He used the thickness-dependent fringe method based on the principle of thin-film white light interferometry, which is known for explaining the changing colors of a soap bubble as its thickness varies. Since then, different techniques have been deployed over two decades to quantify tear film thickness: interferometry based on wavelength-dependent fringe, confocal microscopy, and spectral domain optical coherence tomography (OCT). These methods lack in their ability to provide simultaneous measurements of both the lipid and aqueous layers, or they measure at a single point and are unable to spatially quantify the tear film dynamics.
In spectral domain OCT, the convention is to perform a fast Fourier transform followed by a peak detection technique to extract thickness information. The axial resolution of this method is fundamentally limited by the width of the axial point spread function (PSF), which is in the order of a micron in state-of-the-art systems, thus to date OCT has been used to measure the total thickness of the lipid and aqueous layers combined.